Online content may be received from various first-party or third-party sources. In general, first-party content refers to the primary online content requested or displayed by a user's device. For example, first-party content may be a webpage requested by the client or a stand-alone application (e.g., a video game, a chat program, etc.) running on the device. Third-party content, in contrast, refers to additional content that may be provided in conjunction with the first-party content. For example, third-party content may be a public service announcement or advertisement that appears in conjunction with a requested webpage (e.g., a search result webpage from a search engine, a webpage that includes an online article, a webpage of a social networking service, etc.) or within a stand-alone application (e.g., an advertisement within a game). More generally, a first-party content provider may be any content provider that allows another content provider (i.e., a third-party content provider) to provide content in conjunction with that of the first-party.